I am working on a new app with Clutter, which acts as a digital photo frame (since there doesn't seem to be one in the repos). It currently has two transition modes: fade to black and crossfade. It is set to fade to black by default because there is a bug in crossfade mode where it flashes the new picture before fading to it, and I don't know how to fix this yet (I only learned Clutter about 4 hours ago!).
To run, extract to somewhere where you know where it is, and in a terminal run:
where the first argument is the number of seconds to display each image for, the second is the folder containing the photos and the third is the mode (either "black" or "fade".)

Launcher, settings GUI etc. coming soon.